Field of the Invention
This invention relates to measuring equipment, and more particularly to an electronic circuit for a dimension-measuring device having magnetoresistive electrodes, of the type including an electronic feed circuit supplying at least one feed voltage intended to feed a network of magnetoresistive electrodes and a measuring circuit having at least one input intended to be connected to the network and supplying at least one magnitude dependent upon the resistance of at least one of the magnetoresistive electrodes of the network.
The invention further relates to a dimension-measuring device having magnetoresistive electrodes and to a method of measuring by means of such a device of the type including the steps of feeding Wheatstone bridges with feed voltages and determining the position of a sensor along a scale with the aid of two dephased signals received at the outputs of the Wheatstone bridges.
Electronic devices for measuring length or angular position, e.g., in industry, must generally satisfy a number of partially contradictory requirements. They must furnish sufficient precision and resolution and be usable in environments subject to vibration or to pollution such as dust, oil, or humidity. Also expected of such sensors are easy integration into compact apparatus, without significant adjustment or adaptation, high-speed measurement, and low power consumption, all at the lowest possible cost.
Various types of measuring devices, based upon different physical principles, have been developed to satisfy these various requirements. In particular, measuring systems utilizing capacitance variations caused by the movement of a sensor opposite a scale have been used to a great extent in portable equipment such as gauges, for example. Such devices must be kept quite clean in order to operate and are therefore not very suitable for operation in an environment which is humid or subject to the spraying of lubricant or cutting oil, for example.
Devices for measuring length based upon the magnetoresistive electrode principle offering much greater resistance to soiling have been proposed, e.g., in German Patent No. 4,233,331 to IMO. The device described there includes a sensor provided with a network of magnetoresistive electrodes connected so as to define two Wheatstone bridges. The sensor is mounted on a slide and can move opposite a magnetized scale having a magnetization period .lambda..
Movement of the sensor opposite the scale causes a modification of the magnetic field applied to the various magnetoresistive electrodes of the sensor, and thus a change in their resistance. By applying a voltage to the Wheatstone bridges, an electric signal is produced at their outputs as a periodic function of the position of the sensor along the scale.
The two Wheatstone bridges are made up of four magnetoresistive electrodes dephased by .lambda./2. The corresponding electrodes of each bridge occupy positions dephased by .lambda./4. The electrodes of the two bridges are mixed. The aforementioned patent further suggests the use of barberpole structures which allow the direction of the current vector I to be modified. As the resistance of a magnetoresistive electrode is a function of the angle between the magnetization vector and the current vector, the barberpole structures allow control of the direction and amplitude of the variation in resistance of the electrodes caused by the movement of the sensor.
Each arm of the Wheatstone bridge is made up of a single magnetoresistive electrode which must be wide enough to react to the relatively small magnetic fields generated by the scale. The resistance of the arms of the bridge is therefore low, and sizable currents circulate across the measuring bridges. Consequently, the power consumption of this device is high.
U.S. Pat. No. 5,386,642 to Heidenhain describes a sensor in which the electrodes are organized into Wheatstone bridges, each arm of which is made up of several magnetoresistive electrodes of the same phase and connected in series. Thus, the resistance of the arms of the bridges is greater, allowing the power consumption to be appreciably reduced. However, the consumption of this type of sensor remains too high to contemplate its use in electrically autonomous equipment such as portable precision gauges.